This invention relates to a metallurgical lance and a metallurgical apparatus including the lance. The metallurgical lance according to the invention is particularly suited to the introduction of oxygen or other gases from above into a bath of molten metal.
One use of the lance according to the invention is in steelmaking. Most steel is made today by blowing or injecting oxygen from above into a vessel containing molten iron. An example of such a steelmaking process is the so-called xe2x80x9cLDxe2x80x9d process in which oxygen is injected into the molten metal from above at high velocity. Another example is the xe2x80x9cLD-ACxe2x80x9d process in which oxygen is injected into the molten metal with powdered lime.
In these examples the metallurgical lance is typically capable of delivering oxygen to a steelmaking vessel capable of holding up to 300 tonnes or more of steel. Such a vessel is sometimes called a xe2x80x9cconverterxe2x80x9d. Initially, the lance is positioned from 2 to 4 meters above the level of the metal, and oxygen is blown from the lance at a relatively low velocity vertically downwards into the molten metal so as to produce a foaming slag on the surface of the melt. The resulting slag plays a key role in removing phosphorus from the molten metal. Later, the lance is lowered to within 1 m of the surface of the metal and oxygen is injected at a higher velocity which results in greater penetration of oxygen into the molten metal.
The metallurgical lance is designed to survive in a very aggressive oxidising and particle filled environment and to meet these needs, typically the lance head is made of copper, has more than one outlet orifice for oxygen, and is water cooled. Often the head of the lance has three or four outlet orifices, or more, for the injection of oxygen into the molten metal. The oxygen is typically supplied to the lance at a pressure of up to 15 bar and supersonic exit velocities greater than Mach 2 can thereby be achieved if each outlet orifice is being formed as a venturi.
Even though they are water cooled the lances have a short working life, typically lasting for some 350 to 450 heats.
High oxygen exit velocities from the lance are needed so as to achieve good penetration of the oxygen into the bath of the molten metal. As the oxygen leaves the lance at supersonic velocity it creates a suction force that draws the surrounding atmosphere into the oxygen jet. The jet therefore loses velocity as it spreads. Accordingly, the oxygen enters the molten metal with a velocity significantly lower than that at which it leaves the lance. Further, nitrogen impurity is introduced into the molten metal and can have a deleterious effect on the quality of the steel.
EP-A-1 041 341 addresses the problem of loss of oxygen velocity by proving a plurality of supersonic oxygen jets with a single flame shroud. The shroud reduces the amount by which the oxygen jets diverge before they enter the molten metal, and thereby inhibits the loss of velocity endured by the jets as they pass from the lance to the surface of the molten metal. The resulting oxygen jets are sometimes described as being xe2x80x9ccoherentxe2x80x9d in the sense that they do not significantly diverge.
Such an arrangement does however have a number of disadvantages. Firstly, a supply of fuel to the lance is required in order to form the flame shroud. Since the lance may need to be positioned up to say, 30 meters above floor level, considerable engineering difficulties are added. Secondly, the head of the lance needs to be provided with additional passages for the fuel and an oxidant (typically oxygen) in order to support combustion of the fuel. This adds to the complexity and hence cost of the head. Thirdly, providing a common shroud for a plurality of oxygen jets, results in imperfect shrouding and an incomplete approach to obtaining perfect coherence. Analogous problems occur in other metallurgical processes which use at least one jet of oxygen or other gas supplied from above.
Other references disclose shielding or shrouding a central gas jet ejected from a metallurgical lance, but with a shrouding gas stream of ambient temperature gas. For example, GB-A-1 446 612 discloses employing a lance with an annular insert in each of its oxygen outlets. The oxygen flow is divided by the insert into a central stream and an outer annular stream. The arrangement is such that the annular stream issues from the lance with a radially outward component of velocity. The purpose of the modification to the lance is to confine damage from splashing to the annular insert which is readily replaceable.
GB-A-1 227 876 relates to a metallurgical lance provided with an acoustic resonator in the path of the gas exiting from the lance.
U.S. Pat. No. 4,730,784 relates to a gas nozzle which may form part of a metallurgical lance. The nozzle is designed so as to make it possible to vary the Mach number of the gas independently of its flow rate. To this end, the nozzle is provided with a variable throat. In one embodiment, there are no moving parts and the effective size of the throat is varied by the application to the main gas jet of a subsonic ring of gas. In this embodiment, the main gas jet expands out of a Laval nozzle.
EP-A-0 214 902 relates to a complex metallurgical lance which employs separate outlet passages communicating with a common chamber. However, the passages are not in a spatial arrangement such that gas issuing from one shrouds that issuing from the other.
WO-A-00/28097, on the other hand, relates to a lance which employs a shrouding gas to reduce the rate of attenuation of a central supersonic gas jet.
Of these references, therefore, only WO-A-00/28097 relates to a metallurgical lance which employs a shrouding gas to reduce the rate of attenuation of a central supersonic gas jet. WO-A-00/28097 does not however address the question of how to supply the gas to the central jet and the shrouding stream in a controlled manner.
According to the present invention there is provided a metallurgical lance for introducing gas from above into a volume of molten metal in a vessel, the lance including a head having at least one gas ejector formed therein, wherein the ejector or at least one of the ejectors comprises a Laval nozzle surrounded by a shrouding gas passage, both the Laval nozzle and the shrouding gas passage communicating at their proximal ends with a common gas supply chamber, wherein the shrouding gas passage communicates with the common gas chamber via a first annular orifice member.
The present invention also provides metallurgical apparatus including a metallurgical lance.
The metallurgical lance according to the present invention does not require a separate supply of shrouding gas and therefore circumvents engineering problems associated with such a supply. Each nozzle is provided with its own individual shroud. Further, the metallurgical lance according to the invention does not provide any undue manufacturing problems. The orifice member enables a predetermined proportion of the incoming gas to be diverted to the shrouding gas passage. The size, shape and number of the orifices can, for example, be selected so as to determine the proportion of the gas that is supplied from the common gas supply chamber to the shrouding gas passage. Typically this proportion is from 5% to 20% of the gas supplied to the Laval nozzle depending on its dimensions. For small nozzles, the proportion can be higher, say, up to 50%.
The shrouding gas passage may communicate with the common gas chamber via a first annular orifice plate.
The shrouding gas passage may be defined by a sleeve coaxial with the Laval nozzle. Such an arrangement facilitates manufacture of a metallurgical lance according to the invention.
The orifice plate is preferably demountably attached to the sleeve. One advantage of such an arrangement is that if it is necessary to vary the relative proportions of gas flow through the Laval nozzle and gas flow through the shrouding gas passage, this can be readily achieved by substituting the orifice plate with one having a different percentage of its annular area open; the greater the open area, the greater the proportion of gas that flows from the gas supply chamber to the shrouding gas passage. Alternatively, the metallurgical lance according to the invention may include means for varying the proportion of the annular area of the orifice plate that is open to the common gas supply chamber. For example, the lance may include a second orifice plate with a position which is adjustable relative to the first orifice plate so as to move the orifices of the second plate into and out of registration with the orifices of the first plate.
In an alternative arrangement, the orifice member is integral with the Laval nozzle. In this arrangement the orifices in the orifice member preferably overlap a solid annular plate demountably attached to the proximal end of the Laval nozzle. The degree of overlap determines the area of the orifice member that is effectively open to the common gas supply chamber, and hence the split of the gas between the Laval nozzle and the shrouding gas passage. Accordingly, this split can be selected by choosing a solid annular plate of appropriate size, and can be changed by substituting one solid annular plate for another, the solid annular plates being of different size.
In the alternative arrangement, the Laval nozzle preferably has at least two lugs which engage the wall or walls defining the shrouding gas passage with the Laval nozzle.
Preferably, the distal end of the Laval nozzle is set back relative to the distal end of the ejector. The arrangement helps to lessen any damage to the Laval nozzle that may be caused by splashing molten metal.
The lance preferably has a plurality of gas ejectors although it is possible to use a lance which has a single gas ejector.
In embodiments of the metallurgical lance according to the invention that have a plurality of gas ejectors, all the gas ejectors are preferably essentially the same as each other. The lance typically has a body which is coaxial with the head. There is preferably but a single gas passageway through the body that communicates with the common gas supply chamber. It is however possible to employ different kinds of ejector in the same lance. Thus, there may be one or more conventional ejectors in addition to an arrangement in which one or more Laval nozzles are each provided with their own shrouding gas passage.
The head of the metallurgical lance according to the invention typically has internal passages for the flow of a liquid coolant, for example water.